Numerous methods for the production of hydrogen gas are known in the art. The production of industrial-scale volumes of hydrogen is typically accomplished by application of the steam-methane reforming process, which entails the catalytic reforming of natural gas with steam at elevated temperatures (800-900° C.). This process yields a crude synthesis gas, which is a mixture of hydrogen, carbon monoxide, and carbon dioxide, and the crude synthesis gas is further reacted in a catalytic water-gas shift conversion step to convert carbon monoxide and water to additional hydrogen and carbon dioxide. The shifted synthesis gas is purified to yield a final hydrogen product containing greater than 99 volume % hydrogen.
An alternative process for the production of hydrogen is the partial oxidation of methane to form synthesis gas, which is subsequently shifted if necessary and purified by pressure swing adsorption (PSA). Partial oxidation is known to be highly exothermic. Another alternative process to generate synthesis gas for hydrogen production is autothermal reforming, which is essentially a thermally balanced combination of the steam-methane reforming process and partial oxidation. One considerable drawback associated with these alternative processes is that partial oxidation requires a supply of high purity oxygen gas to the reaction system. Therefore, the use of these processes
requires the additional step of separating air to produce the oxygen gas, and the air separation process increases the capital and operating costs of hydrogen production.
Other methods for hydrogen production are disclosed in U.S. Patent Application Publication No. 2002/0010220, and U.S. Pat. Nos. 5,827,496, 6,007,699, and 6,682,838.
U.S. patent application Ser. No. 11/165,731 discloses a new process for producing hydrogen comprising:                (a) providing a reactor containing a complex metal oxide and a steam-hydrocarbon reforming catalyst, wherein the complex metal oxide is represented by the formula:AxByOn          wherein A represents at least one metallic element having an oxidation state ranging from +1 to +3, inclusive, wherein such metallic element is capable of forming a metal carbonate; x is a number from 1 to 10, inclusive; B represents at least one metallic element having an oxidation state ranging from +1 to +7, inclusive; y is a number from 1 to 10; inclusive and n represents a value such that the complex metal oxide is rendered electrically neutral;        (b) introducing a feed gas containing at least one hydrocarbon and steam into the reactor in a production step, reacting the at least one hydrocarbon and the steam in the presence of the complex metal oxide and the steam-hydrocarbon reforming catalyst under reaction conditions sufficient to form hydrogen and a spent complex metal oxide, and withdrawing from the reactor a product gas comprising hydrogen;        (c) terminating the introduction of the at least one hydrocarbon and depressurizing the reactor and optionally purging the reactor with a purge gas to displace combustible components from the reactor and withdrawing a purge gas effluent therefrom;        (d) regenerating the reactor in a regeneration step by reacting the spent mixed metal oxide and an oxygen source gas under reaction conditions sufficient to regenerate the complex mixed metal oxide;        (e) optionally purging the reactor with a purge gas;        (f) pressurizing the reactor by introducing a pressurizing gas into the reactor at pressure; and        (g) repeating (b) through (f) in a cyclic manner.        
A in the complex metal oxide may represent at least one metallic element selected from the group consisting of elements of Groups 1, 2, and 3, and the Lanthanide elements of the IUPAC Periodic Table of the Elements; and B represents at least one metallic element selected from the group consisting of elements of Groups 4 to 15 of the IUPAC Periodic Table of the Elements. B in the complex metal oxide may be selected from the group consisting of vanadium, chromium, manganese, iron, cobalt, copper, nickel, and mixtures thereof.
The steam-hydrocarbon reforming catalyst may contain one or more components selected from the group consisting of nickel, cobalt, ruthenium, osmium, rhodium, palladium, platinum, iridium, oxides of the foregoing metals, and a catalyst support. The at least one hydrocarbon may be selected from aliphatic hydrocarbons having from 1 to 20 carbon atoms. The at least one hydrocarbon may be methane obtained as a component of natural gas. The molar ratio of steam to methane may range from 1.3:1 to 4:1, inclusive. Alternatively, the at least one hydrocarbon may be provided by pre-reformed natural gas.
The feed gas may comprise methane and the yield of hydrogen produced per mole of methane consumed may be within ±10% of the maximum yield of hydrogen that can be realized at thermoneutral conditions. The yield of hydrogen produced per mole of methane consumed may be within ±5% of the maximum yield of hydrogen that can be realized at thermoneutral conditions.
The production step may be characterized by a production temperature in the range of 350° C. to 9000 or in the range of 650° C. to 750° C., and a production pressure ranging from 1 to 100 atmospheres. The molar ratio of steam to the at least one hydrocarbon may range from 1:1 to 20:1.
As disclosed in U.S. patent application Ser. No. 11/165,731, the purge gas in the two purge steps may be selected from the group consisting of steam, nitrogen, or a mixture thereof. Purge gas in step (c) may be introduced to the reactor to reduce the concentration of combustible gases remaining in the reactor vessel to a safe level for the subsequent addition of air, which is used to regenerate the spent complex metal oxide material. Purge gas in step (e) may be introduced to the reactor to reduce the concentration of oxygen in the reactor vessel to a safe level for the subsequent addition of combustible pressurizing gas. Purging the reactor vessel may be desirable to prevent mixing of combustible gases with high concentrations of oxygen present in the regeneration gas within the reactor vessel, thereby diminishing the possibility for any uncontrolled energy release or temperature excursion.
According to U.S. patent application Ser. No. 11/165,731, the oxygen source for the regeneration step may be selected from the group consisting of air, oxygen, oxygen-depleted air, and mixtures thereof. The production step may be characterized by a production temperature and the regeneration step may be characterized by a regeneration temperature, wherein the regeneration temperature may be equal to or greater than the production temperature and wherein the difference between the regeneration temperature and the production temperature may be 100° C. or less. The regeneration step may be characterized by a regeneration temperature in the range of 450° C. to 900° C.
The production step may be characterized by a production pressure and the regeneration step may be characterized by a regeneration pressure, wherein the pressure of the regeneration step may be less than the pressure of the production step.
Elemental carbon may be deposited during the production step and may be oxidized and removed from the reactor in the regeneration step.
The pressurizing gas may be obtained from the group consisting of hot reactor feed, hot reactor effluent, steam, feed to a pressure swing adsorption system, and product gas. The process may further comprise, prior to purging the reactor in (c), depressurizing the reactor by withdrawing a depressurization gas therefrom. The feed gas may contain up to 25 volume % hydrogen. The feed gas may be pre-reformed natural gas. The process may further comprise cooling the product gas and removing non-hydrogen components therefrom in a pressure swing adsorption process to yield a high-purity hydrogen product comprising at least 99 volume % hydrogen.
According to U.S. patent application Ser. No. 11/165,731, the process may further comprise providing at least one additional reactor containing the complex metal oxide and the steam-hydrocarbon reforming catalyst, and operating the at least one additional reactor by carrying out steps (b) through (f) such that each of the reactors proceeds through the production step (b) during a different time period. A portion of the product gas from the production step may be retained and introduced into the reactor with the feed gas in a succeeding production step.
It has been discovered by the present inventors that purging the complex metal oxide with nitrogen, as disclosed in U.S. patent application Ser. No. 11/165,731, decreases the CO2 retention capacity of the complex metal oxide over time. It would be desirable to retain the CO2 retention capacity of the complex metal oxide.
While purging with nitrogen may negatively affect the CO2 retention capacity of the complex metal oxide, it would still be desirable to eliminate the possibility for any uncontrolled energy release or unacceptable temperature excursion.
As hydrogen production is energy intensive, it would be desirable to increase the energy efficiency of the hydrogen production process that uses complex metal oxides. Purging with steam and/or nitrogen may decrease the energy efficiency of the process.
Known processes for the generation of hydrogen gas from hydrocarbons thus have associated drawbacks and limitations. There is a need in the field of hydrogen generation for improved process technology for the generation of hydrogen gas by the reaction of methane or other hydrocarbons with steam without certain of the limitations associated with known processes. This need is addressed by the embodiments of the present invention described below and defined by the claims that follow.